1. Field of the Invention
The present invention relates to a pumping unit for evacuating tubes and in particular television tubes. To obtain a hard vacuum in the order of 10.sup.-7 torr in a television tube, a two-stage pumping device is generally used comprising a first stage with a mechanical vacuum pump and a second stage with an oil vapour flux diffusion vacuum pump.
2. Description of the Prior Art
It is known for the evacuating of television tubes to be effected by arranging the tubes on frames installed on castors; the frames are driven by a chain through a tunnel furnace in which the vacuum degassing of the tubes is effected by heating them to a temperature in the order of 400.degree. C. Only the top part of the frame carrying the tube is brought into the furnace.
During the vacuum degassing operation, means must be provided for preventing simultaneous ingress of oil molecules passing from the secondary pump into the tube in which a vacuum is being generated. For this purpose, the diffusion pump body and a baffle placed above the pump on its inlet side are vigorously cooled by a flow of pumped water. The water pump must be powerful enough to keep the walls of the diffusion pump at a sufficiently low temperature for substantially all the oil to be condensed at the level of the walls. This requires a fairly heavy device which consumes a great quantity of water and has a surprisingly high consumption of electricity. Moreover, the water supplied to the device causes progressive obstruction and scaling of its supply pipes.
The present invention provides a two stage pumping unit for evacuating tubes, comprising a frame supporting an air-cooled mechanical vacuum pump as a first stage connected in series with an oil diffusion pump as a second stage by a discharge pipe, the diffusion pump having an inlet for receiving the stem of a tube to be evacuated, a seal surrounding the inlet and an oil vapour baffle disposed between the inlet and a body of the pump, the body being enclosed in a main housing, the discharge pipe which connects the pumps in series having a rising portion above the outlet of the oil diffusion pump, and the unit having forced draft generator means drawing air from a relatively cool region of the frame and connected to force a cooling draft into the main housing of the diffusion pump, over the rising portion of the discharge pipe, over the baffle and over the seal surrounding the inlet.
It will be seen that preferred embodiments of this pump have various advantages over existing pumps. The consumption of water, as well as the civil engineering expenses required by the construction of the input and output pipes, are avoided.
Moreover, the scaling and blocking up of the water pipes are also avoided.
Since each pumping frame is provided with its own two-stage pumping device and its own fans for providing a forced air draft, possible breakdowns are limited to individual frames and need not cause the stopping of all of the pumping frames, as occurs in water-cooled frames, in which a breakdown of the water supply in any one of the frames stops all the frames.
Lastly, the consumption of electricity used for cooling the installation is reduced substantially by half.